Acids, Bases and Salts Class X Important Questions and Answers

1.What is efflorescence? Give an example?
Ans. It in the process of the loss of molecules of water of crystallization from a substance when exposed to air for example


2. Why alkalis like sodium hydroxide and potassium hydroxide should not be left exposed to air?

Ans. Alkalis should not be left exposed to air because they are hygroscopic in nature and absorb moisture from atmosphere in which they get dissolved.

3.Why does not an acid show any acidic behavior is the absence of water?

Ans. An acid gets ionized only in aqueous solution which means in the presence of water. Also it releases H + ions and show acidic behavior in the presence of water

4.While diluting an acid, why it is recommended that the acid should be added to water and not water to the acid?

Ans. Addition of water to acid is an exothermic reaction. If we add water to acid lot of heat is produced that may breaks the glass container or sprout to burns the person adding it. But when acid is added to water with constant stirring, the heat produced is absorbed by water and no harm occurs

5.How is concentration of hydroxide ions (OH - ) affected when excess base is dissolved in a solution of sodium hydroxide?

Ans. Excess base dissolved in a solution of sodium hydroxide will release more hydroxide (OH - ) ions. Therefore, concentration of hydroxide ions (OH - ) will increase

6. Equal lengths of magnesium ribbons are taken in test tubes A and B. hydrochloric acid is added to test tube A, while acetic acid is added to test B. In which test tube will the fizzing occur more vigorously and why?

Ans. HCl is stronger acid than CH 3 COOH. Therefore, H + ions concentration in test tube A will be more than that in test tube B. hence, reaction will take place faster in test tube A than in test tube B. so, fizzing will occur more vigorously in test tube B.

7.Plaster of Paris should be stored in moisture-proof container. Explain why?

Ans.This is because the presence of moisture can cause slow setting of plaster of paris by bringing about its hydration.This will make the POP useless 8. A yellow powder X gives a pungent smell if left open in air. It is prepared by the Reaction of dry compound Y with chlorine gas. It is used for disinfecting drinking Water. Identify X and Y. and write the reaction involved. Ans.The yellow powder is bleaching powder and Y is calcium hydroxide the reaction is as follows Ca(OH)2 + Cl2----------->CaOCl2 +H2O

8.What will you observe when:

(i) Red litmus is introduced into a solution of sodium sulphate.
(ii) Methyl orange is added to dil HCl.
(iii). Blue litmus is introduced into a solution of ferric chloride
Ans. (i) It will undergo any colour change because solution of Na 2 SO 4 is water is almost neutral.

(ii) In the acidic solution, the colour of methyl Orange will change to reddish.

(iii) FeCl 3 solution on reacting with water will form ferric hydroxide and hydrochloric acid. Since the acid is strong, the solution will be acidic. Therefore the colour of blue litmus will change to red.

9. A first aid manual suggests that vinegar should be used to treat wasp sting and baking soda for bee stings.

(a) What does this information tell you about the chemical name of the wasp sting?
(b) If there were no baking soda in the house, what other house hold substances would you use to treat as stings?
Ans. (a) The chemical present in the sting must be base because vinegar (acetic acid) is used to heal or neutralize the effect of wasp stings.
(b) Since bee stings are treated by backing soda which is a base it means they must contain some acid. If baking soda is not available in the house, solution of ammonium hydroxide NH 4 OH can be used for the same purpose.

10. How is plaster of Paris chemically different from gypsum? How may these be inter converted? Write one use of plaster of Paris?

Ans. Plaster of Paris is different from Gypsum as it is prepared from heating gypsum. Plaster of Pairs is a major constituent of surgical bandages used for setting fractured bones. Before applying on fractured bone, it is made wet with water and as a result of hydration it changes into gypsum which keeps the bones in position.
The use of plaster of Paris are- (i) It is used for making modules or casts for toys pottery, ceramics etc.
(ii) In surgical bandages for setting fractured bones.

11. Metal compound ‘A’ reacts with dilute hydrochloric acid to produce efferenvescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction, if one of the compounds formed is calcium chloride.

Ans. As one of the compounds formed is calcium chloride, metal compound ‘A’ is salt of calcium. Burning candle is extinguished by carbon dioxide so carbon dioxide gas is produced by reaction of ‘A’ with hydrochloric acid. Carbon dioxide is produced by action of HCl on carbonate that means ‘A’ is calcium carbonate. CaCO 3 + HCl CaCl 2 + CO 2 + H 2 O

12.A milkman adds a very small amount of baking soda to fresh milk.
(a) Why does he shift the pH of the milk from 6 to slightly alkaline?
(b) Why does this milk take a long time to set a curd?

Ans. (a) The pH of milk changes from 6 to slightly alkaline on addition of a very small amount of baking soda. This is because sodium hydrogen carbonate (baking soda) is basic in nature. This prevents the milk from souring.
(b) Lactic acid formed as a result of fermentation is neutralized by sodium hydrogen carbonate. This prolongs the time taken by milk to set as curd

13.A road tanker carrying an acid was involved in an accident and its contents spilled on the road. At the side of the road iron drain cover began melting and fizzing as the acid ran over them. A specialist was called to see if the acid actually leaked into the nearby river.
(a) Explain why specialist could carry out sample test to see of the river water contains some acid or not
(b) Suggest a better report name for the word ‘melting’
(c) Explain why the drain covers began fizzing as the acid ran over them.

Ans. (a) It can be done by adding a strip of blue litmus paper into a tube containing a small amount of sample water if the colour changes into red, this means that some acid has gone into the river.
(b) The acid has reacted chemically with the drain cover which is usually made of iron. The correct word is corrosion.
(c) Iron reacts with an acid to evolve gas. Since the gas is released immediately accompanied by large number of bubbles Fizzing of detain covers is expected

14.A substance ‘X’ used in the kitchen for making tasty crispy pakoras and is also an Ingredient of antacid. Name the substance ‘X’.
(i) How does ‘X’ help to make cakes and bread soft and spongy. (ii) Is the pH value of solution of ‘X’ is lesser than or greater than 7.0?

Ans.

15. A compound X is bitter in taste. It is a component of washing powder& reacts with dil. HCI to produce brisk effervescence dur to colourless, odourless gas Y wich turns lime water milky due to formation of Z. When excs of CO2 is passed, ilkiness disappears due to formation of P. Identify X, Y, And Z & P

Ans.

16. A metal carbonate X on reacting with an acid gives a gas which when passed through a solution Y gives the carbonate back. On the other hand, a gas G that is obtained at anode during electrolysis of brine is passed on dry Y, it gives a compound Z, used for disinfecting drinking water. Identity X, Y, G and Z.

Ans.

17. A dry pellet of a common base B, when kept in open absorbs moisture and turns sticky. The compound is also a by–product of chloralkali process. Identify B. What type of reaction occurs when B is treated with an acidic oxide? Write a balanced chemical equation for one such solution. Ans. 18. What happens when nitric acid is added to egg shell?
  Ans. When nitric acid is added to egg shell, it dissolves egg shell. Egg shell is composed of calcium carbonate which dissolves in nitric acid. The reaction between nitric acid and calcium carbonate yields calcium nitrate and carbon dioxide gas.